Related to this is the role of time. In particular, the relativistic ED models considered previously in ch. REF , as well as in [1]}[2]}, all commit to a particular foliation, and in doing so, break manifest Lorentz invariance. This situation is somewhat reminiscent of the status of quantum field theory in the early 1930's, shortly after the advent of the new quantum mechanics by Heisenberg, Schrödinger, and Dirac.The biographical book by Silvan Schweber [3]} provides, in chapter one, an excellent overview of the status of quantum field theory at that time. For instance, it was common to construct the canonical commutation relations with respect to a particular time variable; as was done, for example, by Pauli and Heisenberg [4]}[5]}. There, although such theories could be relativistic in content — i.e. the generators satisfy the Poincaré algebra, etc. — they were not manifestly so. In particular, such models failed to make explicit the notion of relative simultaneity, which is surely a hallmark of the special and general theories of relativity.
